A wavelength division multiplexing (WDM) transmission system is known in which a large capacity communication can be performed by multiplexing and transmitting a plurality of optical signals different in wavelength or channel, through a same optical transmission path such as an optical fiber. In the WDM transmission system, the optical signals are transmitted as a wavelength multiplexed optical signal.
Problems are caused of reduction in a signal level of each channel and introduction of noise due to a higher density transmission and a longer transmission distance of the wavelength multiplexed optical signal in relation to an increase in a transmission capacity in recent years. When the signal level (intensity) of the optical signal is reduced or a noise quantity is increased, the optical signal to noise ratio (OSNR) is degraded to reduce transmission quality. In order to prevent the reduction in signal level, an optical repeater is provided on a transmission path to amplify the wavelength multiplexed optical signal. However, when the intensity of the optical signal is excessively strong, the transmission quality is degraded due to the non-linear effect of the optical fiber.
For this reason, in the optical transmission system, the signal level of the optical signal for every channel in the wavelength multiplexed optical signal is monitored by an optical channel monitor, and an optical intensity for every channel is controlled to be constant on the transmission path based on the monitored result. The optical channel monitor is provided with an optical separating unit to separate the wavelength multiplexed optical signal to optical signals for respective wavelengths (for respective channels), and measure the intensity of each of the optical signals and calculate the optical levels (intensities) of the optical signals.
The optical channel monitor is described in Japanese Patent Publication (JP 2003-204302A: Patent Literature 1) and Japanese Patent Publication (JP 2001-44938A: Patent Literature 2). The Patent Literature 1 describes a WDM monitor for determining a correction quantity of a wavelength multiplexed optical signal based on a characteristic of the wavelength multiplexed optical signal and a response characteristic data. Also, the Patent Literature 2 describes a monitor which monitors the number of optical signals, in which a wavelength interval between ports is a half of a wavelength interval (channel interval) between the optical signals in the wavelength multiplexed optical signal.